Method and Apparatus for Characterizing and Enhancing the Dynamic Performance of Machine Tools

ABSTRACT

Disclosed are various systems and methods for assessing and improving the capability of a machine tool. The disclosure applies to machine tools having at least one slide configured to move along a motion axis. Various patterns of dynamic excitation commands are employed to drive the one or more slides, typically involving repetitive short distance displacements. A quantification of a measurable merit of machine tool response to the one or more patterns of dynamic excitation commands is typically derived for the machine tool. Examples of measurable merits of machine tool performance include dynamic one axis positional accuracy of the machine tool, dynamic cross-axis stability of the machine tool, and dynamic multi-axis positional accuracy of the machine tool.

GOVERNMENT RIGHTS

The U.S. Government has rights to this invention pursuant to contractnumber DE-AC05-00OR22800 between the U.S. Department of Energy andBabcock & Wilcox Technical Services Y-12, LLC.

FIELD

This disclosure relates to the field of characterizing and enhancing theperformance of numerically controlled machine tools such as millingmachines, grinding machines, and turning machines. More particularly,this disclosure relates to the dynamic performance of numericallycontrolled machine tools in applications with a requirement for precisepositioning operations between an object being machined or measured anda cutting tool, grinding wheel, or inspection probe.

BACKGROUND

Machine tools are designed to produce movement of an object to bemachined or inspected and/or a cutting tool (or grinding wheel) along atleast one axis of motion, and typically three or more axes. For example,a “3-axis milling machine” typically moves any object to be machinedalong two orthogonal horizontal axes (“X” and “Y”) and moves a cuttingtool spindle along a vertical third axis (“Z”) that is orthogonal to Xand Y. A 4 or 5-axis milling machine adds one or two (respectively)rotary axes. An “A” axis provides a tilt angle around the X axis, a “B”axis provides a tilt angle around the Y axis, and a “C” axis provides arotation around the Z axis. Each A, B, and C axis is orthogonal to theother two tilt axes. While three tilt axes could be added to a threeaxis milling machine to make a 6-axis milling machine, many machiningjobs only require 5 axes of motion, so that is a common configuration.Additional motion axes may be added by providing axes that are parallelto each other but offset by a linear displacement. With the addition ofsuch further axes, typical machine configurations are characterized as7-axis machines and 9-axis machines. In addition, dimensional inspectionmachines closely resemble metal-removal machines except that the cuttingtool (or grinding wheel) is replaced with a measurement probe.

Almost all turning machines (e.g., lathes and boring machines) provideat least two axes of relative motion between an object to be turned anda cutting tool. The “X” axis provides movement of the tool carriageperpendicular to the spindle (horizontally across the bed). Anorthogonal “Y” axis provides vertical movement of a tool toward and awayfrom the bed. An orthogonal “Z” axis provides movement of the carriagetoward or away from the spindle chuck. Each of the X, Y and Z axes of aturning machine are orthogonal to the other two axes. Additional toolpath and/or object motion axes may be provided by tilt axes A, B, and C.Lathe and boring machine tilt axes conform to the same standard as thatfor milling machines: The A axis provides a rotation around the X axis,the B axis provides rotation around the Y axis, and the C axis providesrotation around the Z axis.

Precision machining operations require accurate positioning of a cuttingtool with respect to an object being machined. Existing machine toolperformance analysis techniques are typically based upon positionmeasurements along the X, Y, Z, A, B, or C axes that are taken understatic or slowly moving operational conditions. While such tests mayprovide a useful assessment of some aspects of a machine tool's geometryerrors, machine tools typically operate in modes where either the objectbeing machined and/or the cutting tool move in rapid dynamic patterns.The term “dynamic” as used herein refers to conditions associated withrelative acceleration and deceleration between two or more objects. Therapid dynamic patterns of motion are generated by a computer program,referred to herein as a “part program,” irrespective as to whether it isthe object being machined that is being moved or the cutting tool thatis being moved. The part program generates dynamic excitation commandsthat are provided to a machine tool's motion control system. The term“dynamic excitation commands” refers to motion commands that have anacceleration and/or a deceleration component. The motion controller is aprogrammable device and may be a microprocessor, a programmable logiccontroller, or a computer. The motion control system generates commandsignals that are amplified to drive motors that move various componentsof the machine tool.

Most existing machine tool performance analysis techniques do not detectthe errors associated with a machine's dynamic characteristics, whichare especially important in applications that approach the velocity andacceleration limits of a machine's servo system capabilities or whenerrors may be introduced by limits in the dynamic stiffness of a machinetool's structural frame. What are needed therefore are better methods toassess the accuracy and performance characteristics of machine toolsunder dynamic operational conditions.

SUMMARY

The present disclosure provides a method for assessing the dynamicperformance of a machine tool having a first axis slide that has a firstmotion axis. The method includes the step of electronically instructingthe machine tool to drive the first axis slide along the first axisusing a first pattern of dynamic excitation commands that includedisplacements less than about one-half inch, to generate a firstmachine-tool-response. The method further includes the step of derivinga first quantification of a measurable merit of machine tool responsefrom the first machine-tool-response to the first pattern of dynamicexcitation commands.

Also disclosed herein is a method for assessing a dynamic two axispositional accuracy of a machine tool having a first axis slide having afirst motion axis and having a second axis slide having a second motionaxis that is perpendicular to the first motion axis. The method involveselectronically instructing the machine tool to drive the first axisslide along the first motion axis using a first pattern of dynamicexcitation commands while further electronically instructing the machinetool to drive the second axis slide in a second pattern of dynamicexcitation commands along the second motion axis. As the slides arebeing driven, the first actual motion of the first axis slide along thefirst motion axis and a second actual motion of the second axis slidealong the second motion axis are measured. The method further includesthe step of evaluating the dynamic two axis positional accuracy of themachine tool. This technique may also be applied to machines with morethan two axes.

BRIEF DESCRIPTION OF THE DRAWINGS

Various advantages are apparent by reference to the detailed descriptionin conjunction with the figures, wherein elements are not to scale so asto more clearly show the details, wherein like reference numbersindicate like elements throughout the several views, and wherein:

FIG. 1 illustrates a map of machine tool accuracy.

FIG. 2 is a perspective view of features of a three-axis milling machinewith a sensor nest installed.

FIG. 3 is a perspective view of the three-axis milling machine andsensor nest of FIG. 1, with a cubic reference block installed.

FIGS. 4 and 5 are graphs of sinusoidal dynamic patterns of motion.

FIG. 6 is a perspective view of features of a lathe with a sensor nestinstalled.

FIG. 7 is a perspective view of the lathe and sensor nest of FIG. 5,with a cubic reference block installed.

FIG. 8 is an illustration of a map of a machine tool functionalperformance that relates to the length of the chip that is formed duringthe machining operation.

FIG. 9 is an illustration of a map of machine tool functionalperformance that relates to the surface texture of a workpiece.

FIG. 10 is a somewhat schematic view of a machine tool and ancillaryhardware.

DETAILED DESCRIPTION

In the following detailed description of the preferred and otherembodiments, reference is made to the accompanying drawings, which forma part hereof, and within which are shown by way of illustration thepractice of specific embodiments of systems and methods for assessingand improving the capability of a machine tool. The following detaileddescription presents preferred and other embodiments of such systems andmethods. It is to be understood that other embodiments may be utilized,and that structural changes may be made and processes may vary in otherembodiments.

It is fairly common to assess the static or slow-moving positionalaccuracy and other performance characteristics of a machine tool so thaterror compensations may be built into the programs that drive themachine tool. However, it is often necessary to operate a machine toolin a manner that produces rapid accelerations and decelerations of themachine axes (high-speed cornering, modulated tool-path chip breaking,etc.) and the machine's ability to follow the associated dynamic motioncommands has a direct impact on the quality of the workpiece.Embodiments herein are generally directed toward assessment ofpositional accuracy (or inaccuracies) and other performancecharacteristics of a machine tool under dynamic operating conditionssuch as rapidly changing motion vectors, or high speed translationalmotion conditions, or combinations of those two conditions.

Disclosed herein are methods for characterizing the dynamic performanceof machine tools and using this information to enhance a machine'sperformance. These methods typically employ measurements using acombination of an on-machine “sensor nest” and a “cubic referenceblock.” A sensor nest is a configuration of sensors designed to measurea machine's response to rapidly changing axes position commands. Thesensor nest consists of three orthogonal position sensors that aresecured within the sensor-nest framework so that they do not exhibitmotion relative to each other during the machine testing cycle. Thecubic reference block is a precision cube or rectangle that has highquality, flat, perpendicular surfaces that provide a position referencefor the displacement sensors. Both the sensor nest and the referenceblock have support members that allow them to be securely attached tothe machine tool being tested. Single axis dynamic performancecharacterization may be assessed without using a sensor nest andreference block by measuring the motion of the single axis relative to astationary part of the machine. Alternatively, a somewhat less accuratemethod of determining the dynamic performance of the machine is to usethe machine's axes position transducers to record the machine's responseto the dynamic motion commands. The inaccuracies associated with thisapproach are due to the Abbé offset between the position transducers andthe cutting tool or workpiece and the possibility of mechanicaldeflections occurring outside the position measurement loop. However,the data analysis and machine compensation techniques employed would bethe same for either approach.

When used on a 3-axis machining center, jig grinder, coordinatemeasuring machine, or a similar device with horizontal work slides and avertical spindle/probe axis, the cube may be attached to the verticalaxis and aligned so that the faces are nominally perpendicular to theaxes motions. The sensor nest is then attached to the work slide, in aposition that approximates the location used for workpieces, and alignedso that the sensor axes are parallel with the machine axes. Then themachine axes are moved so that the cube is positioned within the sensornest and the sensors are pointed toward the center of the cube. Thismeans that the individual orthogonal sensors only detect motion that isparallel with the sensor axis; the motion perpendicular to the sensoraxis causes the sensor measurement point to traverse across the cubeface and does not produce a displacement signal. A similar approach maybe used with devices such as gantry machines that carry one or morevertical axes on one of the horizontal axes.

On a lathe, grinding machine, or similar device that attaches theworkpiece to a rotary axis and employs stacked slides to create ormeasure a figure of revolution, the cubic reference block may be locatedon a stationary part of the machine and the sensor nest mounted on theuppermost slide. If the lathe design uses independent slides then theblock may be mounted on one slide and the sensors mounted on a secondorthogonal slide. Multi-axis machines may be evaluated in a similarmanner by mounting the system components on the appropriate machineaxes.

The part program that controls the axes motions during the testing cycleproduces an oscillatory motion, such as a sinusoid or other repeatingpattern, which covers a range of amplitudes and frequencies that arechosen for a particular machine's applications. The comparison of thecommanded axes motions with the sensor measurements provides performanceinformation (much like a Bode Plot) that defines the machine's dynamicperformance capabilities as seen by the mechanical loop between aworkpiece and a cutting tool or measurement probe. FIG. 1 illustrates amap of machine tool accuracy, showing a first axis motion response ratioversion frequency of sinusoidal motion for several oscillation commandamplitudes. This performance data includes the effects of the machine'sservo system performance limitations and the mechanical deflections thatoccur outside the machine's position feedback loop.

The displacement sensors mounted in the sensor nest may be selected froma variety of contact or noncontact sensors as long as the sensors' rangeof travel and frequency response are within the desired range of theaxes oscillations. In addition, the alignment of the sensor nest and thecube measurement artifact to the machine axes does not have to beperfect because the data collection/analysis system records the changesin the machine performance as the frequency of the oscillation commandsis increased (in either incremental, “swept sine,” or other modes) froma relatively slow motion to more challenging oscillations. This allowsthe slow speed data to be used for the correction of alignment errors,as needed.

During typical testing operations, the machine tool is electronicallyinstructed to drive a moveable element (a slide or a spindle) through apattern of dynamic excitation commands of displacements that are lessthan about one-half inch (i.e., about +/−one quarter inch). Alternately,in some embodiments, the dynamic response testing operations may usedisplacements as small as about +/−0.005″ or less. The maximum rate ofdisplacement of a movable element is generally dependent on the size andmass of the machine tool and the capability of the axes servo systemsand the test parameters are selected based on the machine'scharacteristics and the intended application. These rates ofdisplacement (axes velocities) may range from a few thousandths of aninch per second for a diamond machining application to many inches persecond for a high speed milling application. In addition, while thesystem can accommodate step or impulse motion commands, the more commontest waveform is an oscillation signal and in this case, an importanttest parameter is the frequency of the excitation commands. Most machinetools are incapable of responding to oscillation commands above 20 Hz;however, this is not an inherent limitation of the performancemonitoring technique. The only limitations associated with this approachare the data collection rate (typically thousands of Hz on currentsystems) and the resonant frequency of the sensory nest and measurementcube mounting system (typically an order of magnitude or higher than themachine's servo capability.)

FIG. 2 illustrates portions of a three-axis milling machine 10. Thethree-axis milling machine 10 is a computer numerical control (CNC)machine that operates by stepper or servo motors that are driven withmotor controllers under electronic instructions loaded into aprogrammable motion control system. The three-axis milling machine 10has a first slide 14 that moves along an X-axis 18, a second slide 22that moves along a Y-axis 26 and a spindle 30 that moves along a Z-axis34. The X-axis 18, the Y-axis 26, and the Z-axis 34 are orthogonal toeach other. FIG. 2 also illustrates a sample sensor nest 38 that isrigidly mounted to the first slide 14. The sensor nest 38 has a firstproximity sensor 42, a second proximity sensor 46, and a third proximitysensor 50. The exact shape of the sensor nest is unimportant as long asit provides a stable platform for positioning the sensors in thecorrection locations/orientations.

FIG. 3 illustrates a cubic reference block 100 that is rigidly mountedon the spindle 30 of the three-axis milling machine 10. The cubicreference block 100 is an example of a “reference block” as that term isused herein and could be any appropriately-sized rectangular cuboid. Thecubic reference block 100 has three orthogonal faces, 104, 108, and 112.The first face 104 is in a plane that is parallel to the X-axis 18 andthe Z-axis 34, and orthogonal to the Y-axis 26 of the three-axis millingmachine 10. The second face 108 is in a plane that is parallel to theY-axis 26 and the Z-axis 34. The third face 112 is in a plane that isparallel to the X-axis 18 and the Y-axis 26 and orthogonal to the Z-axis34.

Displacement between the sensor nest 38 and the cubic reference block100 along the X-axis 18 is measured by the first proximity sensor 42.Displacement between the sensor nest 38 and the cubic reference block100 along the Y-axis 26 is measured by the second proximity sensor 46.Displacement between the sensor nest 38 and the spindle 30 is detectedby the third proximity sensor 50.

To test the dynamic response of the three-axis milling machine 10, apattern of dynamic excitation commands is directed toward establishing adynamic pattern of motion, which is generally a repetitive cyclicalpattern, but may be a non-repeating “one cycle” pattern. A dynamicexcitation command that establishes a repetitive cyclical patterntypically adds an oscillating motion to a basic tool path. In someembodiments, one of the three movable elements (the first slide 14, thesecond slide 22, and/or the spindle 30) may be held in a stationaryposition while the remaining movable element(s) is (are) moved in apattern of dynamic excitation commands.

FIG. 4 illustrates an example of a first sinusoidal dynamic pattern ofmotion. In any case, with dynamic excitation commands there is adirected change in the slope of an axis displacement vs. time curve. Therate of motion may be constant over portions of the cycle (e.g.,sawtooth) or variable (e.g., sinusoidal). Changing between a firstpattern of dynamic excitation commands and a second pattern of dynamicexcitation commands produces a measurement of a machine's ability torespond to a range of machining conditions such as a change in cuttingspeed, a change in frequency, a change in oscillation amplitude, or achange in waveform.

Typically, when measuring the undesirable cross coupling between axes,the three-axis milling machine 10 is electronically instructed to moveonly one movable element (a “first axis slide”) along a “first motionaxis” while keeping the other two movable elements stationary. While theterm “first axis slide” is used here, it is understood that in athree-axis milling machine the movable element may be the spindle andthe term “axis slide” encompasses the spindle. In other test conditionsused to evaluate a machine's ability to perform dynamic contouringmotions, multiple axes or all of the axes may be moved simultaneously.

To assess the dynamic performance of a machine tool, one or moremoveable elements are typically driven along their motion axis in one ormore patterns of dynamic excitation, and a quantification of ameasurable merit of machine tool response to the one or more patterns ofdynamic excitation commands is typically derived for the machine tool.Examples of measurable merits of machine tool performance where only oneaxis slide is dynamically excited (“one-axis excitation”) includedynamic one axis positional accuracy of the machine tool, dynamiccross-axis stability of the machine tool, workpiece surface finish, andthe ability to generate chips of a desired length while performingmodulated tool-path chip breaking operations. As used herein, the term“modulated tool path” refers to a tool path between (for example) points(x₁, y₁, z₁) and (x₂, y₂, z₂) wherein an oscillation (such as asinusoidal oscillation) is superimposed on the basic tool path as itmoves from point (x₁, y₁, z₁) to point (x₂, y₂, z₂). Examples ofmeasurable merit of machine tool performance when two or more movableelements are dynamically excited (“two-axis or multi-axis excitation”)include dynamic two axis positional accuracy of the machine tool,cross-coupling errors between machine elements, workpiece surfacefinish, and the ability to generate chips of the desired length. Theseterms are described in more detail subsequently herein.

Some methods of assessing the dynamic performance of a machine toolinvolve driving one motion axis in a pattern of short displacementsusing a first pattern of excitation commands, and measuring a firstactual motion of the first axis slide along the first motion axis inresponse to the first pattern of dynamic excitation commands. Typicallythe proximity sensor (i.e., one of the proximity sensors 42, 46, or 50)associated with the axis or the spindle that is the movable elementbeing tested is used to measure actual displacement of the “first axisslide” over time under the first pattern of dynamic excitation. A visualindication of machine tool performance may be provided by graphing thefirst pattern of dynamic excitation commands (planned displacement overtime) and the first actual motion (actual displacement over time) on thesame chart and visually observing the two graphs. Typically the firstactual motion is quantitatively compared with the first pattern ofdynamic excitation commands to establish a first quantification of thedynamic one axis positional accuracy of the machine tool. Such aquantification may be in the form of the calculation of a correctioncoefficient or calculation of a best-fit curve through the first actualmotion position measurements. When the machine tool is configured tomanufacture parts, the results of the performance test may be used toprogram the machine tool with modified process parameters in order tocompensate for positional errors indicated by the positional accuracymeasurements or to modify servo system settings to optimize themachine's response for a particular application.

Another method of assessing the dynamic performance of a machine toolinvolves driving one motion axis in two patterns of short displacementexcitation commands and comparing the results. For example, afterdriving one motion axis in a pattern of short displacements using afirst pattern of excitation commands (and measuring a first actualmotion of the first axis slide along the first motion axis in responseto the first pattern of dynamic excitation commands) the dynamicresponse of a milling machine may be further evaluated by electronicallyinstructing the machine tool to drive the first axis slide along thefirst motion axis using a second pattern of dynamic excitation commandscomprising displacements less than about one-half inch, where the secondpattern is different from the first pattern of dynamic excitationcommands. FIG. 5 illustrates an example of a second sinusoidal dynamicpattern of motion. In this embodiment the second sinusoidal dynamicpattern of motion is approximately twice the frequency of the firstsinusoidal dynamic pattern of motion depicted in FIG. 4. The secondpattern of dynamic excitation commands generates a secondmachine-tool-response. A second quantification of the measurable meritof machine tool performance is then derived from the machine's responseto a second pattern of dynamic excitation commands. Typically the firstquantification of the measurable merit of machine tool performance iscompared with the second quantification of the measurable merit ofmachine tool performance to assess the capability of the machine toolunder different dynamic excitations. Previously presented FIG. 1illustrates such a comparison.

The electronic instructions to the machine tool to drive the first axisslide along the first axis using the first pattern of dynamic excitationcommands and to drive the first axis slide along the first axis usingthe second pattern of dynamic excitation commands may be given in oneinstruction set at the start of the test. There may or may not be apause between execution of the first pattern of dynamic excitationcommands and the second pattern of dynamic excitation commands.

A further example of methods for assessing the dynamic performance of amachine tool for dynamic one axis positional accuracy of the machinetool is a method where the step of deriving a first quantification of ameasurable merit of machine tool performance from the firstmachine-tool-response to the first pattern of dynamic excitationcommands involves measuring a first actual motion of the first axisslide along the first motion axis in response to the first pattern ofdynamic excitation commands, and then comparing the first actual motionwith the first pattern of dynamic excitation commands to establish afirst quantification of the dynamic one axis positional accuracy of themachine tool. In this further example the machine tool is furtherelectronically instructed to drive the first axis slide along the firstmotion axis using a second pattern of dynamic excitation commands, wherethe second pattern is different from the first pattern of dynamicexcitation commands. Then a second quantification of the measurablemerit of machine tool performance is derived from the second pattern ofdynamic excitation commands. Deriving the second quantification of themeasurable merit of machine tool performance involves measuring a secondactual motion of the first axis slide along the first motion axis inresponse to the second pattern of dynamic excitation commands, andcomparing the second actual motion with the second pattern of dynamicexcitation commands to establish a second quantification of the dynamicone axis positional accuracy of the machine tool.

Another important measurable merit of machine tool performance is errormotions that are induced in one axis by the motion of one or more otheraxes. This is described as the dynamic cross-axis stability of themachine tool. If only one moveable element (e.g., the first slide 14 orthe second slide 22 or the spindle 30) is electronically instructed tomove in the setup of FIG. 3, there should be no displacement (i.e., a“null response”) of the cubic reference block 100 with respect to thesensor nest 38 in the other orthogonal directions (the “cross-axes.”)However, there often is some measurable displacement in the otherorthogonal directions, especially under rapid displacement patterns ofthe movable element that is instructed to move. Quantifying such dynamiccross-axis stability is useful in understanding the accuracy of amachine tool. Similarly, different combinations of axes may be heldstationary or deliberately moved to assess various cross-axis stabilityconditions.

In embodiments where a machine tool has a second axis slide having asecond motion axis that is perpendicular to the first motion axis (suchas the milling machine 10 of FIGS. 2 and 3) a measurable merit ofmachine tool performance is dynamic cross-axis stability of the machinetool. Quantifying dynamic cross-axis stability typically involvesmeasuring a first actual motion of the second axis slide along thesecond motion axis in response to a first pattern of dynamic excitationcommands given to the first axis, and then comparing the first actualmotion with an expected null response along the second motion axis toestablish a first quantification of the dynamic cross-axis stability ofthe machine tool.

Another dynamic cross-axis stability measurable merit of machine toolperformance involves driving one axis with two patterns of dynamicexcitation commands, and comparing the results. Presuming that themachine tool has a second motion axis, the machine tool iselectronically instructed to drive the first axis slide along the firstaxis using a first pattern of dynamic excitation commands comprisingdisplacements less than about one-half inch while measuring a firstactual motion of the second axis slide along the second motion axis inresponse to the first pattern of dynamic excitation commands, andcomparing the first actual motion with an expected null response alongthe second motion axis to establish a first quantification of thedynamic cross-axis stability of the machine tool. Then the machine toolis electronically instructed to drive the first axis slide along thefirst motion axis using a second pattern of dynamic excitation commandscomprising displacements less than about one-half inch, where the secondpattern being different from the first pattern of dynamic excitationcommands. A second quantification of the measurable merit of machinetool performance is derived from the second pattern of dynamicexcitation commands.

In a further embodiment the machine tool is electronically instructed todrive the first axis slide along the first axis using a first pattern ofdynamic excitation commands using displacements less than about one-halfinch while measuring a first actual motion of the second axis slidealong the second motion axis in response to the first pattern of dynamicexcitation commands, and comparing the first actual motion with anexpected null response along the second motion axis to establish a firstquantification of the dynamic cross-axis stability of the machine tool.Then a more detailed analysis of dynamic cross-axis stability may bemade by electronically instructing the machine tool to drive the firstaxis slide along the first motion axis using a second pattern of dynamicexcitation commands having displacements less than about one-half inch,where the second pattern is different from the first pattern of dynamicexcitation commands. Further with this method, a second quantificationof the measurable merit of machine tool performance is derived from thesecond pattern of dynamic excitation commands. This derivation includesmeasuring a second actual motion of the second axis slide along thesecond motion axis in response to the second pattern of dynamicexcitation commands; and comparing the second actual motion with theexpected null response on the second motion axis to establish a secondquantification of the dynamic cross-axis stability of the machine tool.

Machine tool performance may also be evaluated by driving two motionaxes in two patterns of dynamic excitation and measuring two-axispositional accuracy. This method typically involves the following steps:

-   -   (a) electronically instructing the machine tool to drive the        first axis slide along the first motion axis using a first        pattern of dynamic excitation commands;    -   (b) while performing step (a), electronically instructing the        machine tool to drive the second axis slide in a second pattern        of dynamic excitation commands along the second motion axis,    -   (c) while performing steps (a) and (b), measuring a first actual        motion of the first axis slide along the first motion axis,    -   (d) while performing steps (a) and (b), measuring a second        actual motion of the second axis slide along the second motion        axis, and    -   (e) evaluating the dynamic two axis positional accuracy of the        machine tool by comparing (1) a first ideal relationship between        the first pattern of dynamic excitation commands and the second        pattern of dynamic excitation commands with (2) the first actual        motion of the first axis slide along the first motion axis and        the second actual motion of the second axis slide along the        second motion axis.

It is important to note that in this embodiment the two patterns ofdynamic excitation commands (steps (a) and (b)) may employ the samepattern executed simultaneously for two movable elements (e.g., thefirst axis slide and the second axis slide). Also, the measurements insteps (c) and (d) may be made in two distinct ways. In the firstapproach, both axes motions are measured and plotted against eachother—for equal amplitude, in phase input commands, the response plotwill be a straight line on a 45 degree angle if the two slides respondidentically to the input. Deviations from the straight line defineerrors in the axes synchronization. A second approach is to use the samedynamic input commands but to also utilize a “bi-axis reference block”that allows the measurement of “out of plane errors.” A “bi-axisreference block is a block that has a face that is non-orthogonal to twoof the motion axes of a machine tool. For example, for equal amplitudeinput commands, a 45 degree reference block may be used as the referenceand a single position sensor used to measure the motion normal to the 45degree surface as the two slides move along their motion axes. A bi-axisreference block is an example of a “reference block” as that term isused herein.

Methods disclosed herein for assessing machine tool performance may beapplied to other machine tools besides milling machines. For example,FIG. 6 illustrates a lathe 150 that has a stationary bed 154 and aspindle 158 and a tailstock 162. The lathe 150 also has a first slide166 that moves along an X-axis 170 and a second slide 174 that movesalong a Z-axis 178. The sensor nest 38 previously described with respectto installation on the three-axis milling machine 10 (FIGS. 2 and 3) isinstalled on the first slide 166.

FIG. 7 illustrates how the cubic reference block 100 (previouslydescribed with respect to FIG. 3) may be installed for use with thesensor nest 38 on the lathe 150. For use with the lathe 150 the cubicreference block 100 is attached to a surface 200 by a mounting assembly204. The surface 200 is secured to the bed 154 such that the surface 200remains stationary, and the cubic reference block 100 remains stationaryby virtue of the connection of the cubic reference block 100 to thesurface 200. This configuration enables assessment of the dynamicperformance of the lathe 150 using the methods described herein for amachine tool in general.

In some embodiments it is beneficial to predict the quality of partsthat will be produced by a machine tool while using specific machiningprocess parameters and/or machine set-up parameters. For example, thesurface texture of a machined surface is a quality indicator that istypically characterized in terms of specific wavelengths of interest onthe part surface, or characterized in terms of a maximum and a minimumsurface profile wavelength. A good indication of the surface finishcapability of a specific machine tool and workpiece may be predicted byelectronically instructing the machine tool to drive the first axisslide along the first motion axis and to drive the second axis slidealong the second motion axis using a first pattern of two-axis dynamicexcitation commands while using the sensor nest and cubic referenceblock to monitor the respective slide motions. Then a first predictionof a first surface finish may be derived from the machine's response tothe first pattern of two-axis dynamic excitation commands. That is,measured errors in the machine's response represent predictable patternsin a surface finish that will be produced by the machine. In reality,further defects in surface finish are introduced by such factors ascutting tool defects. Consequently, (except for the highly unlikelysituation of compensating errors) the actual surface finish that themachine will achieve will be somewhat rougher than what is indicated byjust the measured machine response errors. Further steps in predictingthe surface finish capability of a machine tool may involveelectronically instructing the machine tool to drive the first axisslide along the first motion axis and to drive the second axis slidealong the second motion axis using a second pattern of two-axis dynamicexcitation commands, such that a second prediction of surface texture isobtained or the machine performance is predicted when functioning with asecond set of operating parameters by using the machine's response tothe second pattern of two-axis dynamic excitation commands. In someembodiments the same set of dynamic excitation commands may be used withtwo or more sets of machining process parameters settings to determinethe settings that provide the best performance for a specific task. Asused herein the term “machining process parameters” refers to suchvariables as workpiece feed rate, tool feed rate, pause time for tool orworkpiece settling, turning rotation rate, machine set-up parameters,and such modulated tool path parameters as oscillations (waves) perrevolution (OPR), oscillation command frequency, and oscillationamplitude.

When a lathe is used to turn a surface on a part made of a ductilematerial, the material is removed from the part in the form of chips.When turning parts on a lathe, it is generally desirable that the lengthof the chips be limited because long stringy chips tend to curl aroundthe cutting tool and can damage the machine or the surface of the partbeing machined as well as present a hazard to the machine operator.Consequently lathes may be programmed with modulated cutting tool-pathsin order to induce chip breaking. Errors in the dynamic accuracy of alathe affect chip length. For a lathe (or other turning machine) that isbeing used in a modulated tool-path chip breaking mode of operation, thechip length control capability of the turning machine may be evaluatedby electronically instructing the turning machine to drive the machineaxes using a first pattern of dynamic excitation commands and deriving afirst quantification of a chip length capability. Measured dynamicerrors may be compared with the intended modulated tool path and anassessment of the impact on the chip length can be determined using amodel, such as the one shown in FIG. 8, which relates chip length to amachine's ability to execute specific oscillation commands. Evaluatingan ability of a lathe to generate chips of the desired length is anexample of an evaluation of machine tool functional performance. Tofurther evaluate the chip length control capability of the turningmachine, the method typically involves electronically instructing theturning machine to drive the turning machine axes using additionalpatterns of dynamic excitation commands and deriving a fullquantification of the machine's ability to control the chip length whenusing specific machining process parameters.

FIG. 9 illustrates another example of an encoded map that may be used toevaluate machine tool functional performance. FIG. 9 predicts surfacefinish for a machine tool using the modulated tool-path chip breakingtechnique with combinations of various oscillations per revolution (OPR)and oscillation amplitudes. A particular surface texture may begenerated by selecting various combinations of an OPR and an oscillationamplitude. In practice, a user may employ FIGS. 8 and 9 to determine themodulation parameters needed to deliver a particular chip length andsurface finish. A further step may be to use dynamic machinecharacterization data, such as is shown in FIG. 1, to determine theability of a particular machine tool to execute the necessary modulatedtool-path commands. If a given machine is unable to perform the requireddynamic motions, then it is necessary to modify the machining processparameters and/or the tool-path commands to achieve the needed result.

FIG. 10 is a highly schematic illustration of a system 250 for assessinga capability of a machine tool 254. Only portions of the machine tool254 are illustrated in the highly schematic illustration of FIG. 10. Themachine tool 254 has a first axis slide 258 that moves along a firstmotion axis 262. The system 250 includes a hard drive 266. The harddrive 266 is an example of a data-processor-readable medium. Stored onthe hard drive 266 are data-processor-readable instructions 270 forassessing a capability of a machine tool. The data-processor-readableinstructions 270 include a definition of a measurable merit of machinetool performance and a definition of a first pattern of dynamicexcitation commands representative of a first intended motion anddefinition of a second pattern of dynamic excitation commandsrepresentative of a second pattern of motion.

The system 250 also includes a reference block 274 and a sensor system278 for measuring a relative motion between the sensor system 278 andthe reference block 274. There is a data processor system 282. In someembodiments the data processor system 282 is embedded in the motioncontroller for the machine tool 254. The data processor system 282includes subsystems for:

-   -   (a) reading the data-processor-readable instructions on the        data-processor-readable medium;    -   (b) electronically instructing the machine tool to drive the        first axis slide along the first motion axis using the first        pattern of dynamic excitation commands and to drive the first        axis slide along the first motion axis using the second pattern        of dynamic excitation commands;    -   (c) deriving from the sensor system a first quantification of        the measurable merit corresponding to the first pattern of        dynamic excitation commands;    -   (d) deriving from the sensor system a second quantification of        the measurable merit corresponding to the second pattern of        dynamic excitation commands; and    -   (e) encoding a map of machine tool performance in the        data-processor-readable medium, the map identifying the first        quantification of the measurable merit of machine tool        performance as a function of the first pattern of dynamic        excitation commands and the map identifying the second        quantification of the measurable merit of the machine tool        performance as a function of the second pattern of dynamic        excitation commands.

Some embodiments include a system for improving the performance of amachine tool. The system includes an encoded map (such as encoded map ofFIG. 8 or FIG. 9) of machine tool performance stored in adata-processor-readable medium (such as the hard drive 266 of FIG. 10).An input device, such as a computer workstation is provided foraccepting a machining job objective expressed as a function of at leasta portion of at least one measurable merit of machine tool performance.There is a data processing system, such as the data processor system 282of FIG. 10) that is provided for:

-   -   (f) receiving the machining job objective;    -   (g) reading at least a portion of the encoded map of machine        tool performance;    -   (h) selecting an optimized pattern of dynamic excitation        commands based at least in part upon the encoded map and the        machining job objective; and    -   (i) providing the optimized pattern of dynamic excitation        commands to a machine controller in the machine.    -   The optimized pattern of dynamic excitation commands may be used        to perform sensor/block testing or may be used to enhance some        aspect of machine performance quality such as surface finish or        optimized chip breaking.

In summary, embodiments disclosed herein provide methods for assessingthe dynamic performance of a machine tool that has at least one drivenaxis using at least one pattern of dynamic excitation commands. Variousmeasurable merits of machine tool performance may be derived, such aspositional accuracy, cross-axis error, surface finish, and chip length.

The foregoing descriptions of embodiments have been presented forpurposes of illustration and exposition. They are not intended to beexhaustive or to limit the embodiments to the precise forms disclosed.Obvious modifications or variations are possible in light of the aboveteachings. The embodiments are chosen and described in an effort toprovide the best illustrations of principles and practical applications,and to thereby enable one of ordinary skill in the art to utilize thevarious embodiments as described and with various modifications as aresuited to the particular use contemplated. All such modifications andvariations are within the scope of the appended claims when interpretedin accordance with the breadth to which they are fairly, legally, andequitably entitled.

1. A method for assessing a dynamic performance of a machine tool havinga first axis slide that has a first motion axis, comprising: (a)electronically instructing the machine tool to drive the first axisslide along the first axis using a first pattern of dynamic excitationcommands comprising displacements less than about one-half inch togenerate a first machine-tool-response; and (b) deriving a firstquantification of a measurable merit of machine tool performance fromthe first machine-tool-response to the first pattern of dynamicexcitation commands.
 2. The method of claim 1 further comprising: (c)electronically instructing the machine tool to drive the first axisslide along the first motion axis using a second pattern of dynamicexcitation commands comprising displacements less than about one-halfinch, the second pattern being different from the first pattern ofdynamic excitation commands to generate a second machine-tool-response;and (d) deriving a second quantification of the measurable merit ofmachine tool performance from the second machine-tool-response to thesecond pattern of dynamic excitation commands.
 3. The method of claim 1wherein the measurable merit of machine tool performance comprisesdynamic one axis positional accuracy of the machine tool, and wherein:step (b) comprises (i) measuring a first actual motion of the first axisslide along the first motion axis in response to the first pattern ofdynamic excitation commands, and (ii) comparing the first actual motionwith the first pattern of dynamic excitation commands to establish afirst quantification of the dynamic one axis positional accuracy of themachine tool.
 4. The method of claim 1 wherein the measurable merit ofmachine tool performance comprises dynamic one axis positional accuracyof the machine tool and wherein step (b) comprises: (i) measuring afirst actual motion of the first axis slide along the first motion axisin response to the first pattern of dynamic excitation commands, and(ii) comparing the first actual motion with the first pattern of dynamicexcitation commands to establish a first quantification of the dynamicone axis positional accuracy of the machine tool; and wherein the methodfurther comprises: (c) electronically instructing the machine tool todrive the first axis slide along the first motion axis using a secondpattern of dynamic excitation commands comprising displacements lessthan about one-half inch, the second pattern being different from thefirst pattern of dynamic excitation commands; (d) deriving a secondquantification of the measurable merit of machine tool performance fromthe second pattern of dynamic excitation commands, comprising: (i)measuring a second actual motion of the first axis slide along the firstmotion axis in response to the second pattern of dynamic excitationcommands, and (ii) comparing the second actual motion with the secondpattern of dynamic excitation commands to establish a secondquantification of the dynamic one axis positional accuracy of themachine tool.
 5. The method of claim 1 wherein the measurable merit ofmachine tool performance comprises dynamic cross-axis stability of themachine tool and wherein the machine tool has a second axis slide havinga second motion axis that is perpendicular to the first motion axis, andstep (b) comprises: (i) measuring a first actual motion of the secondaxis slide along the second motion axis in response to the first patternof dynamic excitation commands, and (ii) comparing the first actualmotion with an expected null response along the second motion axis toestablish a first quantification of the dynamic cross-axis stability ofthe machine tool.
 6. The method of claim 1 wherein the measurable meritof machine tool performance comprises dynamic cross-axis stability ofthe machine tool and wherein the machine tool has a second motion axisslide having a second motion axis that is perpendicular to the firstmotion axis, and: step (b) comprises: (i) measuring a first actualmotion of the second axis slide along the second motion axis in responseto the first pattern of dynamic excitation commands, and (ii) comparingthe first actual motion with an expected null response along the secondmotion axis to establish a first quantification of the dynamiccross-axis stability of the machine tool; and wherein the method furthercomprises: (c) electronically instructing the machine tool to drive thefirst axis slide along the first motion axis using a second pattern ofdynamic excitation commands comprising displacements less than aboutone-half inch, the second pattern being different from the first patternof dynamic excitation commands; and (d) deriving a second quantificationof the measurable merit of machine tool performance from the secondpattern of dynamic excitation commands.
 7. A method for assessing adynamic multi-axis positional accuracy of a machine tool having a firstaxis slide having a first motion axis and having a second axis slidehaving a second motion axis that is perpendicular to the first motionaxis, comprising: (a) electronically instructing the machine tool todrive the first axis slide along the first motion axis using a firstpattern of dynamic excitation commands; (b) while performing step (a),electronically instructing the machine tool to drive the second axisslide in a second pattern of dynamic excitation commands along thesecond motion axis, (c) while performing steps (a) and (b), measuring afirst actual motion of the first axis slide along the first motion axis,(d) while performing steps (a) and (b), measuring a second actual motionof the second axis slide along the second motion axis, (e) evaluatingthe dynamic multi-axis positional accuracy of the machine tool bycomparing (1) a first ideal relationship between the first pattern ofdynamic excitation commands and the second pattern of dynamic excitationcommands with (2) the first actual motion of the first axis slide alongthe first motion axis and the second actual motion of the second axisslide along the second motion axis.